The semiconductor industry continues to develop lithographic technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet (“EUV”) light (also sometimes referred to as soft x-rays) is generally defined to be electromagnetic radiation having wavelengths of between 10 and 120 nm. EUV lithography is currently generally considered to include EUV light at wavelengths in the range of 10-14 nm, and is used to produce extremely small features, for example, sub-32 nm features, in substrates such as silicon wafers. These systems must be highly reliable and provide cost effective throughput and reasonable process latitude.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements, e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc., with one or more emission line(s) in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the desired line-emitting element, with a laser pulse at an irradiation site. The target material may contain the spectral line-emitting element in a pure form or alloy form, for example, an alloy that is a liquid at desired temperatures, or may be mixed or dispersed with another material such as a liquid.
A droplet generator heats the target material and extrudes the heated target material as droplets which travel along a trajectory to the irradiation site to intersect the laser pulse. Ideally, the irradiation site is at one focal point of a reflective collector. When the laser pulse hits the droplets at the irradiation site, the droplets are vaporized and the reflective collector causes the resulting EUV light output to be maximized at another focal point of the collector.
In earlier EUV systems, a laser light source, such as a CO2 laser source, is on continuously to direct a beam of light to the irradiation site, but without an output coupler so that the source builds up gain but does not lase. When a droplet of target material reaches the irradiation site, the droplet causes a cavity to form between the droplet and the light source and causes lasing within the cavity. The lasing then heats the droplet and generates the plasma and EUV light output. In such “NoMO” systems (called such because they do not have a master oscillator) no timing of the arrival of the droplet at the irradiation site is needed, since the system only lases when a droplet is present there.
However, it is necessary to track the trajectory of the droplets in such systems to insure that they arrive at the irradiation site. If the output of the droplet generator is on an inappropriate path, the droplets may not pass through the irradiation site, which may result in no lasing at all or reduced efficiency in creating EUV energy. Further, plasma formed from preceding droplets may interfere with the trajectory of succeeding droplets, pushing the droplets out of the irradiation site.
Some prior art NoMo systems accomplish such tracking of the droplets by passing a low power laser through lenses to create a “curtain,” i.e., a thin plane of laser light through which the droplets pass on the way to the irradiation site. When a droplet passes through the plane, a flash is generated by the reflection of the laser light of the plane from the droplet. The location of the flash may be detected to determine the trajectory of the droplet, and a feedback signal sent to a steering mechanism to redirect the output of the droplet generator as necessary to keep the droplets on a trajectory that carries them to the irradiation site.
Other prior art NoMo systems improve on this by using two curtains between the droplet generator and the irradiation site, one closer to the irradiation site than the other. The flash created as a droplet passed through the first curtain may, for example, be used to control a “coarse” steering mechanism, and the flash from the second curtain used to control a “fine” steering mechanism, to provide greater control over correction of the droplet trajectory than when only a single curtain is used.
More recently, NoMO systems have generally been replaced by “MOPA” systems, in which a master oscillator and power amplifier form a source laser which may be fired as and when desired, regardless of whether there is a droplet present at the irradiation site or not, and “MOPA PP” (“MOPA with pre-pulse”) systems in which a droplet is sequentially illuminated by more than one light pulse. In a MOPA PP system, a “pre-pulse” is first used to heat, vaporize or ionize the droplet and generate a weak plasma, followed by a “main pulse”which converts most or all of the droplet material into a strong plasma to produce EUV light emission.
One advantage of MOPA and MOPA PP systems is that the source laser need not be on constantly, in contrast to a NoMO system. However, since the source laser in such a system is not on constantly, firing the laser at an appropriate time so as to deliver a droplet and a main laser pulse to the desired irradiation site simultaneously for plasma initiation presents additional timing and control problems beyond those of prior systems. It is not only necessary for the main laser pulses to be focused on an irradiation site through which the droplet will pass, but the firing of the laser must also be timed so as to allow the main laser pulses to intersect the droplet when it passes through that irradiation site in order to obtain a good plasma, and thus good EUV light. In addition, in a MOPA PP system, the pre-pulse must target the droplet very accurately, and at a slightly different location than the irradiation site.
What is needed is an improved way of controlling both the trajectory of the droplets and the timing with which they arrive at the irradiation site, so that when the source laser is fired it will irradiate the droplets at the irradiation site.